RFID devices, commonly referred to as RFID tags, are well known in the art for a variety of applications. When enquired by a reader emitting radio frequency waves, the RFID tag responds typically with a unique identifier. Commonly, such tags comprise a wireless communication device in form of an RFID chip and an antenna circuit. While RFID chips are being miniaturised by improvement in semiconductor fabrication, smaller antenna circuits are difficult to achieve due to their small size relative to the wavelength of radio signal to be received from the reader. A distinction is made between active and passive tags, depending on whether the tag comprises a power source, or is powered by the received radio signal.
RFD devices exist for both the low frequency (LF), high frequency (HF), and the ultra high frequency (UHF) bands. Due to the lower frequencies in the LF and HF range, such devices typically require larger antenna circuits than UHF range devices. A number of different antenna designs seek to overcome different design challenges, however often thus targeting the antenna for specific applications. For instance, reading distance, i.e. the distance between a reading unit and the tag, may often be sacrificed in order to obtain a small antenna.
Typically, existing marking tags for animals including RFID devices operate in the LF or HF range. Typically, they are operated in a near field range, where the magnetic field from a reader is significant. In a far field, the electric field from the reader will dominate, and the magnetic field be vanishing. Typically, such tags require antennas in form of a wire spool, e.g. with a diameter around 10 cm. However, for animal marking tags, smaller diameter spools are used, resulting in a short maximum reading distance. Commonly, such wire spools are made of copper, which is of environmental concern. Also, should the wire be broken at any paint of such a spool, e.g. by an animal bite, the antenna will no longer function. Additionally, LF devices must commonly be interrogated individually, otherwise collision will occur.
Typically, REID devices operating in the HF range comprise a planar antenna spool. Such devices may e.g. be found in library books and in passports. However, such devices may not typically be interrogated from a side, i.e. In the plane of the antenna. Thus, a specific orientation of the tag is needed to ensure good reader performance. Like in the case for an LF device, a HF device would easily be destroyed in the event of even a minor damage to the antenna circuit.
Electronic marking devices are known in the art. For example, US 2005/093677 discloses an RFID device for marking purposes with improved reading distance. The RFID device may be configured for operation in the UHF range of radio frequencies. The RFID device comprises an antenna structure that is made from a thin sheet material, and that has an elongated slot that facilitates increased readability of the RFID device. An RFID interposer is connected across peripheral ends of the elongated slot. However, the RFID device of US 2005/093677 is difficult to secure to an item in a rugged environment, such as encountered when marking animals. Furthermore, the RFID device of US 2005/093677 does not provide a simple means for tuning the device to different electromagnetic environments as typically encountered when dealing with different applications.
Further, WO 01/37215 A discloses a mobile data carrier for the electronic marking of items, wherein the mobile data carrier comprises a transponder made from a surface wave component with a slot antenna. The surface wave element is placed on the slot and connected to either side of the slot by means of respective electrical contacts, wherein the input impedance of the surface wave element determines the position of the surface wave element on the slot. However, the data carrier of WO 01/37215 A does not provide a simple means for tuning the device to different electromagnetic environments as typically encountered when dealing with different applications. Adaption of the mobile data carrier of WO 01/37215 A either requires a new antenna structure, where the length of the antenna slot is cut specifically for each different application, or requires a bulky construction with a back plate mounted with distance elements, in order to make the data carrier less sensitive to an application specific electromagnetic environment.
EP 0 941 656 A discloses a device for marking/identifying animals, the device comprising a carrier provided with an electronic transponder consisting of at least one electronic component and one antenna winding.